The etching process of a printed circuit board (PCB) is as follows: applying an etchant on a pre-developed copper-clad laminate and etching away the unprotected, non-conductor part of the printed circuit board, in order to form a circuit. The etching of the non-conductor part utilises redox reaction between the etchant and copper. Said developed copper-clad laminate is made in the previous process and has a pattern.
In the early period, ferric chloride etchant was used in the printed circuit board industry. This etchant uses ferric chloride (FeCl3) as a copper etching agent. The etching rate of FeCl3 is fast and the cost is relatively low. However, as the etchant does not contain any artificially added oxidant which can oxidise FeCl2, ferric chloride is reduced to ferrous chloride (FeCl2) without further regeneration of the ferric chloride. Ferrous chloride is not capable of etching copper in the copper etching process. The concentration of ferric chloride in the etchant is not constant, therefore the etching rate is unstable, and it is difficult to ensure etching quality. Thus, this etchant is seldom applied at present.
At present, the etchant which is widely used in the industry is an acidic cupric chloride etchant. In comparison with the ferric chloride etchant, this etchant has constant etching rate, is easy to control, has relatively high etching quality, and its cost is relatively low.
In the prior art, the acidic cupric chloride etchant employs cupric chloride (CuCl2) as the copper etching agent to etch the copper foil on a printed circuit board. The Cu2+ of cupric chloride can oxidise metal copper on the surface of the printed circuit board to Cu+:Cu+CuCl2→Cu2Cl2 
The cuprous chloride (Cu2Cl2) generated is insoluble in water, and is adhered to the surface of the copper and can be advantageously used as a banking agent to reduce the level of etch undercut of the circuit. However, when the precipitation is excessive, the etching rate will be seriously affected.
Thus, generally, there is still a need to incorporate an acidic oxidising system, such as hydrochloric acid+sodium chlorate (NaClO3) or hydrochloric acid+hydrogen peroxide (H2O2) into the cupric chloride etchant to constitute a recycling system, in order to regenerate cupric chloride.
The general regeneration reaction of cupric chloride is Cu2Cl2+2HCl+[O]→2CuCl2+H2O.
The regeneration reaction of cupric chloride using sodium chlorate is 3Cu2Cl2+6HCl+NaClO3→6CuCl2+NaCl+3H2O.
The regeneration reaction of cupric chloride using hydrogen peroxide is Cu2Cl2+2HCl+H2O2→2CuCl2+2H2O.
Generally, a salt capable of providing chloride ions (such as NaCl or NH4Cl) can also be added to act as an etchant additive to accelerate the regeneration of cupric chloride.
As the components in the etchant changes continuously during the etching process, an automatic detection and charging control machine is generally required in industrial production to detect a variety of parameters of the etchant, such as oxidation-reduction potential (ORP), specific density, concentration of dissociated hydrogen ions and the like, and control the charging amount of each component of the etchant in a real-time manner, in order to achieve automatic continuous regeneration of the acidic cupric chloride etchant and thus maintaining a stable etching rate of the etchant. Generally, the etchant can be separated into the following components, and each component may be charged according to corresponding specific parameters:
1. cupric chloride;
2. sub-etchant: generally comprising HCl and an optional etchant additive (such as NaCl);
3. oxidant: being H2O2 or NaClO3 generally; and
4. water.
In the process of preparing the etchant, cupric chloride is firstly added into the sub-etchant to allow the concentration of copper ions in the solution to arrive at a prescribed value which is set according to production requirements.
The mass of added cupric chloride is calculated according to formula 1:
                                          molar            ⁢                                                  ⁢            mass            ⁢                                                  ⁢            of            ⁢                                                  ⁢                          CuCl              2                                            molar            ⁢                                                  ⁢            mass            ⁢                                                  ⁢            of            ⁢                                                  ⁢            copper            ⁢                                                  ⁢            ion                          =                                                            mass                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                                  CuCl                  2                                ⁢                                                                  ⁢                to                ⁢                                                                  ⁢                be                ⁢                                                                  ⁢                added                            ⁢                                                          ⁢                                                          ⁢                              per                ⁢                                                                  ⁢                liter                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                sub                ⁢                                  -                                ⁢                etchant                                                                    mass                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                copper                ⁢                                                                  ⁢                ion                ⁢                                                                  ⁢                to                ⁢                                                                  ⁢                be                            ⁢                                                          ⁢                                                          ⁢                              added                ⁢                                                                  ⁢                per                ⁢                                                                  ⁢                liter                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                sub                ⁢                                  -                                            ⁢                                                          ⁢                                                          ⁢              etchant                                =                                    mass              ⁢                                                          ⁢              of              ⁢                                                          ⁢              pre              ⁢                              -                            ⁢              added              ⁢                                                          ⁢                              CuCl                2                                                                                                ⁢                                                mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  copper                  ⁢                                                                          ⁢                  ions                                ⁢                                                                  ⁢                                  corresponding                  ⁢                                                                          ⁢                  to                  ⁢                                                                          ⁢                                      CuCl                    2                                                  ⁢                                                                  ⁢                                                                  ⁢                added                                                                        (                  Formula          ⁢                                          ⁢          1                )            
wherein the molar mass of cupric chloride is 134.5 g/mol, the molar mass of copper ion is 63.5 g/mol, and the mass of copper ion to be added into per litre of sub-etchant is the prescribed concentration of copper ions in the etchant (unit: g/L).
Assuming that the prescribed mass of copper ion to be added into per litre of sub-etchant is A g, the mass B of cupric chloride to be added into per litre of sub-etchant, according to formula 1, is (A×134.5÷63.5) g.
After the concentration of copper ions in the sub-etchant arrives at the prescribed value, an automatic detection and charging control machine on a PCB etching production line is used for detecting and controlling the ORP, the concentration of dissociated hydrogen ions and other parameters, such that the sub-etchant and/or the oxidant and/or water can be appropriately supplemented, to allow the concentration of copper ions, the concentration of dissociated hydrogen ions and the ORP of the obtained solution to achieve numerical values preset according to production requirements and obtain the etchant as required for the etching process. A hydrometer on the automatic detection and charging control machine is used for measuring the specific density of the etchant obtained, and a specific density numerical control meter is set according to the specific density value measured by the hydrometer. Alternatively speaking, the numerical value of control points for charging water is set to allow the salt concentration in the etchant to be constant, thereby preventing precipitation in the production process. Afterwards, t he etchant is sprayed onto the surface of the printed circuit board to start the etching operation.
During the etching process, the etchant reacts with copper continuously, and the multiple parameters of the etchant change continuously. In order to achieve a stable etching rate as well as requirements for etching quality, the automatic detection and charging control machine needs to be employed throughout the etching process, to appropriately supplement the sub-etchant, the oxidant and the water in a real-time manner. This keeps the various parameters of the etchant (including the concentration of dissociated hydrogen ions, ORP and specific density) constant at the prescribed values, so that the concentrations of each component in the etchant remain constant. The details thereof are described as follows:
1. Monitoring water supplement: as the etchant reacts with copper on the printed circuit board, the copper content in the etchant is gradually increased, causing the gradual increase of specific density of the etchant. When the specific density exceeds the prescribed value, the automatic detection and charging control machine adds water into the etchant by controlling the corresponding water charging pump, to reduce the specific density of the etchant. The salt concentration of the etchant is thus kept constant, precipitation is prevented and the concentration of copper ions in the etchant is controlled.
2. Monitoring sub-etchant supplement: as HCl is consumed in the regeneration reaction of cupric chloride, the concentration of dissociated hydrogen ions in the etchant is gradually decreased. When the concentration of dissociated hydrogen ions is lower than the prescribed value, the automatic detection and charging control machine adds the sub-etchant into the etchant by controlling the corresponding sub-etchant charging pump, to increase the concentration of dissociated hydrogen ions in the etchant.
3. Monitoring oxidant supplement: as the copper etching agent (i.e. cupric chloride) in the etchant is consumed by reacting with copper on the printed circuit board, the ORP of the etchant is gradually decreased. When the ORP is lower than the set value, the automatic detection and charging control machine adds the oxidant into the etchant by controlling the corresponding oxidant charging pump, to increase the ORP value. The said oxidant participates in the regeneration reaction of the copper etching agent.
As the degree of integration of electronic products increases, the demand for printed circuit boards with fine-line circuits is ever increasing. Generally, “fine-line circuits” refers to circuits with line width and line spacing of below 60 μm, which requires the use of higher etching quality etchants during etching. The etching quality is frequently discussed in terms of “etch factor”, which indicates the level of etch undercut. In the etching process, the etchant not only etches downwards, but also etches in the left and right directions. This etching in the left and right directions by the etchant is referred to as “etch undercut”. The etch factor (K) is the ratio of etching depth (D) to undercut width at one side (C) (namely K=D/C). When the etching depth D is the same, a larger etch factor indicates a narrower undercut width; as the two side walls of the etched circuit is more vertical, the etching quality is better.
A major factor affecting the undercut width is the amount of banking agent. Banking agent refers to a substance which is adhered to the two side walls of the circuit and can weaken the degree of attack on the two side walls by the etchant. As described above, when the acidic cupric chloride etchant reacts with copper, the cuprous chloride (Cu2Cl2) generated can be used as a banking agent. When the amount of the banking agent is too small, the level of etch undercut is high and the etch factor is small.
In summary, the regeneration reaction of cupric chloride can be effectively accelerated by increasing the concentration of dissociated hydrogen ions in the etchant, so that the etching rate can be increased. In order to meet large-scale production efficiency, the etching rate needs to be 30 μm/min or higher. An acidic cupric chloride etchant with a relatively high concentration of dissociated hydrogen ions, which can also be referred to as high-acidity cupric chloride etchant, needs to be used. The so-called “high acidity” refers to a “high-acidity system”, which is an acidic cupric chloride etchant with concentration of dissociated hydrogen ions [H+] higher than 2.0M. In contrast, an acidic cupric chloride etchant with concentration of dissociated hydrogen ions [H+] of 2.0M or lower is a low-acidity system' etchant. As the etching rates of low-acidity system etchants are too low, they cannot be applied to large-scale production. The acidic cupric chloride etchants currently used in the PCB industry all belong to the high-acidity system. However, the existing high-acidity cupric chloride etchants have the following disadvantages:
1. Irritating acidic odour will be given off at working temperature (about 50° C.), thereby affecting physical health of the production staff and damaging the environment.
2. When the concentration of hydrogen ions is high, a large amount of the Cu2Cl2 banking agent is converted to CuCl2, thereby causing relatively serious etch undercut; in the case of the high-acidity system, most of the etch factors obtained by etching fine circuits with copper foil thickness of 1 oz, line width and line spacing of 50 μm are less than 3. Serious undercut results in the printed circuit board being prone to short-circuit, which poses potential safety risks to both the workplace and the production staff.
3. In the case of using sodium chlorate as the oxidant for cuprous chloride, when the automatic detection and charging control machine fails, which causes the input of excessive sodium chlorate, chlorine gas will be produced. In mild circumstances, mild chlorine gas poisoning is likely to take place, affecting physical health of the production staff and polluting the environment. In severe circumstances, the large amount of chlorine gas produced may cause moderate to severe chlorine gas poisoning, which lead to casualties.
In the prior art, it is difficult to balance between etching rate and etching quality to meet the etching production requirements of printed circuit boards with fine-line circuits.